Shower Faucet Handles Come Off - Yet the Coverings just spin

Hi
I feel this should be a simple solution; however, I have made several searches and have not come across the answer. I'm in the process of replacing washers in a double faucet shower stall. The Cap and Handles come of easily, yet the covering (shown in the picture) only spins when turning in a counter-clockwise motion. Accessing the back will require tearing up a wall. I'm hoping someone can offer a better solution. Thanks in Advance